In cellular systems, handover control is carried out for switching between connecting cells in response to a user's traveling. In the handover control, a mobile station evaluates transmission conditions in the connecting cell and adjacent cells, and the handover is activated based on the evaluation.
The term “adjacent cells” used herein includes at least one of four types of cells: cells operated by the same base station apparatus and at the same frequency as the currently connecting cell, cells operated by the same base station apparatus as the currently connecting cell but at a frequency different from the currently connecting cell, cells operated at the same frequency as the currently connecting cell but by a base station apparatus different from the currently connecting cell, and cells operated by a different base station apparatus and at a different frequency from the currently connecting cell. In order to perform the handover to the adjacent cells, mobile stations must evaluate transmission conditions in the connecting cells and the adjacent cells.
In radio communication systems in compliance with HSDPA (High Speed Downlink Packet Access) or other standards, a mobile station frequently transmits radio channel conditions of a connecting cell (referred to as CQI (Channel Quality Indicator) hereinafter) to a MAC (Media Access Control) entity in a connecting base station apparatus for link adaptation to fast fading such as transmission power control and AMC (Adaptive Modulation and Coding). In the HSDPA, for example, a mobile station may measure a ratio of the amount of receiving chip energy to the amount of interference power (Ec/IO) for a common pilot channel transmitted from a base station apparatus and transmit the ratio as the CQI at a period of 2 ms or its integer multiple.
Also, the mobile station may be required to measure the Ec/IO for the common pilot channel or pathloss in the connecting cell and adjacent cells and transmit the measurements in RRC (Radio Resource Control) periodically. In this case, the mobile station may periodically transmit the measurements to a radio network control apparatus in RRC messages. Also, if a reporting condition is specified in the RRC, the mobile station may compare the reporting condition with the measurements. Then, if the reporting condition is satisfied, the mobile station transmits the satisfaction in RRC messages.